MICROMETER, an instrument usually fitted to a telescope, in the focus of the object-glass, for measuring small angles or distances, as the apparent diameter of the planets. The general principle of this instrument is, that it moves a fine wire, parallel to itself, in the plane of the picture of an object, formed in the focus of a telescope, and thus measures its per pendicular distance from a fixed wire in the same plane.
This instrument was invented about the year 1666 ; and it has, of course, under gone many improvements since that time. Dr. Gascoigne divided the image of an object, in the focus of the object-glass, by the approach of two pieces of metal, ground to a very fine edge ; instead of which, Dr. Hook would substitute two fine hairs, stretched parallel to each other : and two other methods of Dr. Hook, different from this, are described in his posthumous works. An account of several curious observations which Mr. Gascoigne made by the help of his mi crometer, particularly in measuring the diameter of the moon and other planets, may be seen in the Philosophical Trans actions, vol. xlviii.
De la Hire, in a discourse on the era of the inventions of the micrometer, pen dulum clock, and telescope, read before the Royal Academy of Sciences in 1717, makes PI Huygens the inventor of the micrometer. That author, he observes, in his " Observations on Saturn's Ring, &c." published in 1659, gives a method of finding the diameters of the planets by Means of a telescope, viz by putting an object, which he calls a virgule, of a size proper to take in the distance to be mea. sured, in the focus of the convex object glass : in this case, says he, the smallest object will be seen very distinctly in that place of the glass. By such means, he adds, he measured the diameter of the planets, as he there delivers them.
De la Hire says, that there is no me. thod more simple or commodious for ob serving the digits of an eclipse, than a net in the focus of the telescope. These, he says, were usually made of silken threads; and for this particular purpose six concentric circles had also been used, drawn upon oiled paper ; but he advises to draw the circles on very thin pieces of glass, with the point of a diamond. He also gives some particular directions to assist persons in using them. In another memoir, he shews a method of making use of the same net for all eclipses, by using a telescope with two object-glasses, and placing them at different distances from each other.
IL Cassini invented a very ingenious method of ascertaining the right ascen sions and declinations of stars, by fixing four cross hairs in the focus of the tele scope, and turning it about its axis so as to make them move in a line parallel to one of them. But the later improved mi. crometers will answer this purpose with greater exactness. Dr. Maskeline has published directions for the use of it, ex tracted from Dr. Bradley's papers, in the Philos. Trans. vol. lxii.
Dr. Derharn tells us that his microme ter is not put into a tube, as is usual, but is contrived to measure the spectres of the sun on paper, of any radius, or to measure any part of them. By this means he can easily, and very exactly, with thi help of a fine thread, take the declination of a solar spot at any time of the day ; and, by his half-seconds watch, measure the distance of the spot from either limb of the sun.
J. A. Segner proposed to enlarge the field of view in these micrometers, by making them of a considerable extent, and having a moveable eye-glass, or seve ral eye-glasses, placed opposite to differ ent parts of it. He thought, however, that two would be quite sufficient, and he gives particular directions how to make use of such micrometers in astronomical observations.
A considerable improvement in the mi crometer was communicated to the Royal Society, in 1743, by Mr. 9. Savary an account of which, extracted from the mi nutes by Mr. Short, was published in the Philos. Trans. for 1753. The first hint of such a micrometer was suggested by M. Roemer in 1675: and M. Bougner proposed a construction similar to that of K. Savary in 1748. The late Mr. Doi land made a further improvement in this kind of micrdmeter, an account of which was given to the Royal Society by Mr. Short, and published in the Philos. Trans. vol. xlviii. Instead of two object-glasses he used only one, which he neatly cut into two simi.circles, and fitted each se mi-circle in a metal frame, so that their diameters sliding in one another, by means of a screw, may have their centres so brought together as to appear like one glass, and so form one image ; or, by their centres receding, may form two Images of the same object: it being a property of such glasses, for any segment to exhi bit a perfect image of an object, although not so bright as the whole _glass would give it. If proper scales are fitted to this instrument, sheaving how far the centres recede, relative to the focal length of the glass, they will also spew how far the two parts of the same objects are asunder, re. lative to its distance from the object glass ; and consequently give the angle under which the distance of the parts of that object are seen. This divided ob ject.glass micrometer, which was applied by the late Mr. Dolland to the object end of a reflecting telescope, and has been with equal advantage adapted by his son to the end of an achromatic telescope, is of so easy use, and affords so large a scale, that it is generally looked upon by astro nomers as the most convenient and exact instrument for measuring small distances in the heavens. However, the common micrometer is peculiarly adapted for mea swing differences of right ascension and declination of celestial objects, but less convenient and exact formeasuring their absolute distances ; whereas the object glass micrometer is peculiarly fitted for measuring distances, though generally supposed improper for the former pur pose. But Dr. Maskelyne has found that this may be applied with very little trou ble to that purpose also; and he has fur nished the directions necessary to be fol lowed when it is used in this manner. The addition requisite for this purpose is a cell, containing two wires, intersect ing each other at right angles, placed. in the focus of the eye-glass of the telescope, and moveable round about by the turn ing of a button. For the description of this apparatus, with the method of ap plying and using it, see Dr. Maskelyne's paper on the subject, in the Philos. Trans. vol. lxi.